Change all time formatting to UTC and off of time.RFC3339Nano
Change all time formatting to UTC and off of time.RFC3339Nano

Prometheus has issues parsing RFC3339Nano timestamps if the year has more than 4 digits, in addition it is the second-pass parse attempt. Since this is a client library and the interface is a `time.Time` it makes sense that we pick the clearest simplest format-- so I propose we use the `model.Time` representation of time in our communications to prometheus. This (1) removes the issues with timezones in those queries going downstream and (2) completely works around this #614 issue as the parsing mechanism in prometheus can handle those times in this format.

Related to #614

Signed-off-by: Thomas Jackson <2d00bc2c74768cf27c5a78623e81f1b7f1ca2826@gmail.com>

m3db/prometheus_client_golang,prometheus/client_golang